We shall conclude that the black blood is almost as powerful a stimulus as the red blood to the inner surface of those cavities, which usually contain the latter only: the reason why they are the first to be arrested in their action, is, because they do not receive so large a quantity of blood as the others. Notwithstanding what I have said, I do not entirely reject the idea of the red-blooded ventricle being not excitable by the black blood. It may indeed be less excitable by this sort of blood, than by the other; but I believe that the preceding considerations will reduce this difference of excitement to a mere trifle. The following, however, is an experiment where such difference would appear to be very manifest. If a stop cock be adapted to the trachea, and an artery opened, the blood will blacken, and continue for some time to be thrown out with its ordinary force, but at last the jet will gradually grow weaker. If, after this, the air be admitted, the blood will almost immediately become red and its jet be visibly augmented. In this case the sudden augmentation appears at first to depend upon the simple contact of the red fluid with the sides of the aortic ventricle, since it has not had the time to penetrate the tissue of the heart; but let things be a little attentively examined, and it will soon be seen that this impetuosity of impulse, depends on the movements of inspiration and expiration, to which the animal is obliged, on the admission of air into the lungs. The heart excited at its exterior, and perhaps a little compressed by these movements, is the occasion of such phenomenon, and expels the blood with a force which is far beyond that which results from its habitual contractions. What I have here advanced is proved, by the manifest diminution of the jet, as soon as once the lungs have taken on their accustomed degree of action. Besides, the influence of a series of full expirations may be manifested without dividing the trachea. Open the carotid and precipitate the respiration of the animal by tormenting it (for pain will constantly have this effect) and the jet of blood will be visibly increased. The same augmentation may be artificially produced, by suddenly compressing the parietes of the thorax. These experiments succeed best on animals already weakened by the loss of a certain quantity of blood. In the ordinary state of health however, a series of strong expirations will not be found to quicken the pulse. Of this I cannot assign the reason. From what we have now said, it follows, that the experiment, in which, on the opening of the cock of the syringe, the blood is thrown out with augmented force, is not so conclusive as might at first be imagined. I confess that it embarrassed me much for many days, I hold then my former conclusions to be good. In the red and cold-blooded animals, the action of the lungs has not so immediate a connexion with that of the heart, as it has in the red and warm blooded animals.